Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. Data are written onto and read from a rapidly rotating recording disk by means of a magnetic head transducer assembly that flies closely over the surface of the disk. The escalating requirements for high linear recording density and increasingly smaller disk drives impose increasingly demanding requirements on thin film magnetic recording media in terms of coercivity, remanence, coercivity squareness, low medium noise and narrow track recording performance. Considerable effort has been expended in recent years to produce magnetic recording media having high linear recording densities while satisfying such demanding requirements, particularly for longitudinal recording. However, it is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high density magnetic rigid disk medium.
In order to realize ultra-high linear recording density, magnetic recording media with high coercivity and small Mrt (magnetic remanence×film thickness) are needed. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically noncoupled grains. Medium noise typically measured as the signal to noise ratio (SNR), is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Therefore, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and typically comprises a non-magnetic substrate 10 having sequentially deposited thereon an underlayer 11, such as chromium (Cr) or a Cr-alloy, a magnetic layer 12, typically comprising a cobalt (Co) alloy, a protective overcoat 13, typically containing carbon, and a lubricant topcoat 14. Underlayer 11, magnetic layer 12 and protective overcoat 13 are typically deposited by sputtering techniques. The Co alloy magnetic layer normally comprises polycrystallities epitaxially grown on the polycrystal Cr or Cr-alloy underlayer. Conventional magnetic recording media comprise a cobalt (Co)based magnetic alloy layer, such as polycrystalline cobalt-chromium-tantalum (CoCrTa), cobalt-chromium-platinum (CoCrPt), cobalt-chromium-tantalum-platinum (CoCrTaPt) and cobalt-chromium-platinum-boron (CoCrPtB). The underlayer 11 schematically illustrated in FIG. 1 can comprise a plurality of seedlayers and/or underlayers. Although layers 11 through 14 are shown formed on top of substrate 10, it should be recognized that such layers 11 through 14 are conventionally formed sequentially on both sides of substrate 10.
Major sources of recording noise from magnetic thin film media can be attributed to the zigzag transition boundary due to finite grain size, resulting from bits polarized in opposite directions, and intergranular magnetic coupling. Prior attempts to suppress such noise sources have focused on reducing grain size and isolating grains. Grain size reduction can effectively reduce the magnetic transition width, and thereby reduce noise. However, the reduction in noise benefits to be achieved by grain size reduction is limited by the superparamagnetic limit commonly referred to as the thermal stability limit. Basically, when a grain within a magnetic thin film becomes too small, it begins to loose its ability to resist thermal perturbation and maintain the orientation of its magnetization. Once a ferromagnetic material becomes thermally unstable, it can no longer be employed as a medium for permanent data storage. At any given temperature, the exact grain size at which a ferromagnetic material becomes thermally unstable is to a large extent dependent upon the particular material. For example, typical cobalt (Co)based alloys can remain thermally stable at normal disk drive operating temperatures, i.e., about 75° C., if the grain diameter is not significantly lower than about 7 to about 8 nm.
However, not all grains within a magnetic thin film exhibit the same grain size. In fact, the grain size distribution typically follows a log-normal distribution, as shown in FIG. 2. Accordingly, a reduction in grain size typically means a reduction in the average grain size, i.e., a downward shift of the entire grain size distribution. For a given distribution, there are inevitably some grains that are too small to remain thermally stable at the disk drive operating temperature. These small grains are often referred to as “thermal idiots”. As the overall grain size distribution of a magnetic film shifts downwardly, there will be more and more thermal idiots in the film. When a significant portion of grains become thermal idiots, the film is, manifestly, no longer capable of storing data permanently.
As a result of magnetic coupling between grains, a magnetic switching unit in a thin film media may typically consist of a cluster of grains, which could lead to broad transition even if each individual grain is small. To isolate grains, non-ferromagnetic elements such as chromium (Cr), manganese (Mn) and tantalum (Ta), are usually introduced to the Co-based magnetic alloys. Such non-ferromagnetic elements tend to segregate at the grain boundaries by diffusion. As a result of such segregation, the magnetic interactions among neighboring grains are impaired. In order to increase the effectiveness of grain boundary segregation, the concentration of the non-ferromagnetic elements in the alloy must be sufficiently high to ensure an adequate driving force for diffusion. However, the addition of such non-ferromagnetic elements produces undesirable side effects, such as diluting the magnetic moment within the grain since not all of the non-ferromagnetic atoms diffuse to the grain boundaries. Consequently, the magnetic signal is reduced. Such non-ferromagnetic elements also lower the magnetic anisotropy of the material, leading to lower coercivity and reduced thermal stability. There is, therefore, a fundamental conflict between producing a magnetic recording medium exhibiting lower noise and producing a magnetic recording media exhibiting high thermal stability.
Accordingly, a need exists for a magnetic recording media exhibiting high thermal stability and low media noise and for enabling methodology. There exists a particular need for such media capable of high areal recording density exhibiting high coercivity.